The advantages of needle-free injection devices have been recognized for some time. Some of the advantages of needle-free devices and methods include the absence of a needle which can intimidate a patient and also present a hazard to healthcare workers. In addition, injection using a needle may increase the risk of cross-contamination between patients. Furthermore, with an injection device that employs a needle there is substantial risk of needle breakage in the tissue of a human or animal patient. The injection jet generated by a needle-free device is generally smaller in diameter than a hypodermic needle and thus in certain instances a needle-free injection is less painful than an injection provided by a hypodermic needle device.
Because of these and other advantages of needle-free injection many variations of pneumatic, electronic or spring activated needle-free injection devices have been designed to provide a single injection, or alternatively a series of injections to one or more patients. Most known needle-free injection devices operate by driving the injectable fluid through a fine nozzle with a powered piston to create a fine but high pressure jet of fluid that penetrates the skin. Needle free injection devices are not inherently risk free. For example, it is possible if precautions are not taken, to cause a laceration as opposed to a proper injection with a needle-free device. In addition, it is critical to design a needle-free device with safety features substantially minimizing the risk of inadvertent triggering or injection.
Thus, a great deal of attention has been given to the development of needle-free injection devices and methods which are safe, reliable and easy to use in the field. Needle-free technologies raise certain unique engineering challenges which are likely to be encountered when designing a suitable device. For example, conventional needled syringes are often inexpensive or disposable devices. Thus a large supply of pre-filled syringes can be prepared for large scale inoculation projects. On the other hand, needle-free devices are typically more expensive since these devices require a relatively sophisticated pneumatic, electronic or spring power source, energizing system and triggering system. Although a needle-free device can be designed to accept disposable (or recyclable) needle-free syringes, it can be difficult to quickly and accurately load a pre-filled needle-free syringe into an injection device, particularly without contaminating the injection nozzle. Similarly, it can be difficult to remove a spent needle-free syringe and replace same with an unused syringe quickly, efficiently and in a sterile manner. Thus, known needle-free injection devices can be difficult to use for large scale inoculation projects or in other situations where a significant number of injections are made to a relatively large group of patients.
Safety issues may involve the risk of accidental discharge of a needle-free device. Safety issue can become acute in association with devices that have exposed triggers or devices which include a ram or piston driving mechanism that can extend beyond the housing of the injector. The risk of using these types of devices is similar to the risks associated with the triggers on firearms. Thus, the inadvertent pressing of an exposed and armed trigger can cause the accidental or premature firing of the needle-free injection device.
One class of reliability issue with known needle-free injection devices involves difficulty delivering an entire preselected dosage of injectable liquid into the appropriate tissue of a patient. Dosage reliability issues have a broad spectrum of causes. One significant underlying cause is the difficulty encountered in the creation of a suitable jet or stream of fluid and introduction of this jet into or through the skin of a patient. Preferably, the jet will be a very fine jet that will impact a section of taught skin with much of the energy of the stream being used to penetrate the skin. The elasticity and permeability of a patient's skin can however vary with respect to other patients or across different locations on a patient's body. Another reliability issue concerns difficulty encountered efficiently and accurately pre-filling needle-free syringes to a selected dosage without significant waste of a potentially very limited supply of injectable fluid.
The embodiments disclosed herein are directed toward overcoming one or more of the problems discussed above.